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Copyright © 2004 Pearson Education, Inc.

Copyright © 2004 Pearson Education, Inc.

Chapter 2

Database System
Concepts and
Architecture

Elmasri and Navathe, Fundamentals of Database Systems,
Fourth Edition

Copyright © 2004 Pearson Education, Inc.


Slide 2
-
3

Data Models


Data Model
: A set of concepts to describe the
structure

of a database,

and certain

constraints

that the database should obey.


Data Model Operations
: Operations for
specifying database retrievals and updates by
referring to the concepts of the data model.
Operations on the data model may include
basic
operations

and
user
-
defined operations
.

Elmasri and Navathe, Fundamentals of Database Systems,
Fourth Edition

Copyright © 2004 Pearson Education, Inc.


Slide 2
-
4

Categories of data models


Conceptual

(
high
-
level
,
semantic
) data models:
Provide concepts that are close to the way many
users
perceive

data. (Also called
entity
-
based

or
object
-
based

data models.)


Physical

(
low
-
level
,
internal
) data models:
Provide concepts that describe details of how data
is stored in the computer.


Implementation

(
representational
) data models:
Provide concepts that fall between the above two,
balancing user views with some computer storage
details.

Elmasri and Navathe, Fundamentals of Database Systems,
Fourth Edition

Copyright © 2004 Pearson Education, Inc.


Slide 2
-
5

History of Data Models



Relational Model
: proposed in 1970 by E.F. Codd (IBM),
first commercial system in 1981
-
82. Now in several
commercial products (DB2, ORACLE, SQL Server,
SYBASE, INFORMIX).


Network Model
: the first one to be implemented by
Honeywell in 1964
-
65 (IDS System). Adopted heavily
due to the support by CODASYL (CODASYL
-

DBTG
report of 1971). Later implemented in a large variety of
systems
-

IDMS (Cullinet
-

now CA), DMS 1100 (Unisys),
IMAGE (H.P.), VAX
-
DBMS (Digital Equipment Corp.).


Hierarchical Data Model
: implemented in a joint effort by
IBM and North American Rockwell around 1965. Resulted
in the IMS family of systems. The most popular model.
Other system based on this model: System 2k (SAS inc.)

Elmasri and Navathe, Fundamentals of Database Systems,
Fourth Edition

Copyright © 2004 Pearson Education, Inc.


Slide 2
-
6

History of Data Models


Object
-
oriented Data Model(s)
: several models have been
proposed for implementing in a database system. One set
comprises models of persistent O
-
O Programming
Languages such as C++ (e.g., in OBJECTSTORE or
VERSANT), and Smalltalk (e.g., in GEMSTONE).
Additionally, systems like O
2,
ORION (at MCC
-

then
ITASCA), IRIS (at H.P.
-

used in Open OODB).


Object
-
Relational Models
: Most Recent Trend. Started
with Informix Universal Server. Exemplified in the latest
versions of Oracle
-
10i, DB2, and SQL Server etc. systems.

Elmasri and Navathe, Fundamentals of Database Systems,
Fourth Edition

Copyright © 2004 Pearson Education, Inc.


Slide 2
-
7

Hierarchical Model


ADVANTAGES:


Hierarchical Model is simple to construct and operate on


Corresponds to a number of natural hierarchically organized
domains
-

e.g., assemblies in manufacturing, personnel
organization in companies


Language is simple; uses constructs like GET, GET
UNIQUE, GET NEXT, GET NEXT WITHIN PARENT etc.


DISADVANTAGES:


Navigational and procedural nature of processing


Database is visualized as a linear arrangement of records


Little scope for "query optimization"

Elmasri and Navathe, Fundamentals of Database Systems,
Fourth Edition

Copyright © 2004 Pearson Education, Inc.


Slide 2
-
8

Network Model


ADVANTAGES:


Network Model is able to model complex relationships and
represents semantics of add/delete on the relationships.


Can handle most situations for modeling using record types
and relationship types.


Language is navigational; uses constructs like FIND, FIND
member, FIND owner, FIND NEXT within set, GET etc.
Programmers can do optimal navigation through the database.


DISADVANTAGES:


Navigational and procedural nature of processing


Database contains a complex array of pointers that thread
through a set of records.


Little scope for automated "query optimization”

Elmasri and Navathe, Fundamentals of Database Systems,
Fourth Edition

Copyright © 2004 Pearson Education, Inc.


Slide 2
-
9

Schemas versus Instances


Database Schema
: The
description

of a database.
Includes descriptions of the database structure and
the constraints that should hold on the database.


Schema Diagram
: A diagrammatic display of
(some aspects of) a database schema.


Schema Construct
: A component of the schema
or an object within the schema, e.g., STUDENT,
COURSE.


Database Instance
: The actual data stored in a
database at a
particular moment in time
. Also
called
database state

(or
occurrence
).

Elmasri and Navathe, Fundamentals of Database Systems,
Fourth Edition

Copyright © 2004 Pearson Education, Inc.


Slide 2
-
10

Database Schema Vs.
Database State


Database State:

Refers to the content of a database
at a moment in time.


Initial Database State:

Refers to the database when
it is loaded


Valid State:

A state that satisfies the structure and
constraints of the database.


Distinction


The
database schema

changes
very infrequently
. The
database state

changes
every time the database is updated.


Schema

is also called
intension
, whereas
state

is called
extension
.

Elmasri and Navathe, Fundamentals of Database Systems,
Fourth Edition

Copyright © 2004 Pearson Education, Inc.


Slide 2
-
11

Three
-
Schema Architecture


Proposed to support DBMS characteristics
of:


Program
-
data independence
.


Support of
multiple views

of the data.

Elmasri and Navathe, Fundamentals of Database Systems,
Fourth Edition

Copyright © 2004 Pearson Education, Inc.


Slide 2
-
12

Three
-
Schema Architecture


Defines DBMS schemas at
three levels
:



Internal schema

at the internal level to describe
physical storage structures and access paths. Typically
uses a
physical

data model.


Conceptual schema

at the conceptual level to describe
the structure and constraints for the
whole

database for
a community of users. Uses a
conceptual

or an
implementation

data model.


External schemas

at the external level to describe the
various user views. Usually uses the same data model
as the conceptual level.

Elmasri and Navathe, Fundamentals of Database Systems,
Fourth Edition

Copyright © 2004 Pearson Education, Inc.


Slide 2
-
13

Three
-
Schema Architecture

Mappings

among schema levels are needed
to transform requests and data. Programs
refer to an external schema, and are mapped
by the DBMS to the internal schema for
execution.

Elmasri and Navathe, Fundamentals of Database Systems,
Fourth Edition

Copyright © 2004 Pearson Education, Inc.


Slide 2
-
14

Data Independence


Logical Data Independence
: The capacity
to change the conceptual schema without
having to change the external schemas and
their application programs.


Physical Data Independence
: The capacity
to change the internal schema without
having to change the conceptual schema.

Elmasri and Navathe, Fundamentals of Database Systems,
Fourth Edition

Copyright © 2004 Pearson Education, Inc.


Slide 2
-
15

Data Independence

When a schema at a lower level is changed,
only the
mappings

between this schema
and higher
-
level schemas need to be
changed in a DBMS that fully supports data
independence. The higher
-
level schemas
themselves are
unchanged
. Hence, the
application programs need not be changed
since they refer to the external schemas.

Elmasri and Navathe, Fundamentals of Database Systems,
Fourth Edition

Copyright © 2004 Pearson Education, Inc.


Slide 2
-
16

DBMS Languages


Data Definition Language

(
DDL
): Used by the
DBA and database designers to specify the
conceptual schema

of a database. In many
DBMSs, the DDL is also used to define internal
and external schemas (views). In some DBMSs,
separate
storage definition language

(
SDL
) and
view definition language

(
VDL
) are used to
define internal and external schemas.


Elmasri and Navathe, Fundamentals of Database Systems,
Fourth Edition

Copyright © 2004 Pearson Education, Inc.


Slide 2
-
17

DBMS Languages


Data Manipulation Language

(
DML
):
Used to specify database retrievals and
updates.


DML commands (
data sublanguage
) can be
embedded

in a general
-
purpose programming
language (
host language
), such as COBOL, C
or an Assembly Language.


Alternatively,
stand
-
alone

DML commands can
be applied directly (
query language
).

Elmasri and Navathe, Fundamentals of Database Systems,
Fourth Edition

Copyright © 2004 Pearson Education, Inc.


Slide 2
-
18

DBMS Languages


High Level
or

Non
-
procedural
Languages:

e.g., SQL, are
set
-
oriented
and
specify what data to retrieve than how to
retrieve. Also called
declarative

languages.


Low Level
or

Procedural Languages:
record
-
at
-
a
-
time;

they specify
how

to
retrieve data and include constructs such as
looping.

Elmasri and Navathe, Fundamentals of Database Systems,
Fourth Edition

Copyright © 2004 Pearson Education, Inc.


Slide 2
-
19

DBMS Interfaces


Stand
-
alone query language interfaces.


Programmer interfaces for embedding DML in
programming languages:


Pre
-
compiler Approach


Procedure (Subroutine) Call Approach


User
-
friendly interfaces:


Menu
-
based, popular for browsing on the web


Forms
-
based, designed for naïve users


Graphics
-
based (Point and Click, Drag and Drop etc.)


Natural language: requests in written English


Combinations of the above

Elmasri and Navathe, Fundamentals of Database Systems,
Fourth Edition

Copyright © 2004 Pearson Education, Inc.


Slide 2
-
20

Other DBMS Interfaces


Speech as Input (?) and Output


Web Browser as an interface


Parametric interfaces (e.g., bank tellers) using
function keys.


Interfaces for the DBA:


Creating accounts, granting authorizations


Setting system parameters


Changing schemas or access path

Elmasri and Navathe, Fundamentals of Database Systems,
Fourth Edition

Copyright © 2004 Pearson Education, Inc.


Slide 2
-
21

Database System Utilities


To perform certain functions such as:


Loading

data stored in files into a database. Includes
data conversion tools.


Backing up

the database periodically on tape.


Reorganizing

database file structures.


Report generation

utilities.


Performance monitoring

utilities.


Other functions, such as
sorting
,
user monitoring
,
data
compression
, etc.

Elmasri and Navathe, Fundamentals of Database Systems,
Fourth Edition

Copyright © 2004 Pearson Education, Inc.


Slide 2
-
22

Other Tools


Data dictionary / repository
:


Used to store schema descriptions and other information such
as design decisions, application program descriptions, user
information, usage standards, etc.


Active

data dictionary is accessed by DBMS software and
users/DBA.


Passive

data dictionary is accessed by users/DBA only.


Application Development Environments and CASE
(computer
-
aided software engineering) tools:


Examples


Power builder (Sybase), Builder (Borland)

Elmasri and Navathe, Fundamentals of Database Systems,
Fourth Edition

Copyright © 2004 Pearson Education, Inc.


Slide 2
-
23

Centralized and Client
-
Server
Architectures



Centralized DBMS:

combines everything
into single system including
-

DBMS
software, hardware, application programs
and user interface processing software.

Elmasri and Navathe, Fundamentals of Database Systems,
Fourth Edition

Copyright © 2004 Pearson Education, Inc.


Slide 2
-
24

Basic Client
-
Server
Architectures


Specialized Servers with Specialized
functions


Clients


DBMS Server

Elmasri and Navathe, Fundamentals of Database Systems,
Fourth Edition

Copyright © 2004 Pearson Education, Inc.


Slide 2
-
25

Specialized Servers with
Specialized functions:



File Servers


Printer Servers


Web Servers


E
-
mail Servers

Elmasri and Navathe, Fundamentals of Database Systems,
Fourth Edition

Copyright © 2004 Pearson Education, Inc.


Slide 2
-
26

Clients:



Provide appropriate interfaces and a client
-
version
of the system to access and utilize the server
resources.


Clients maybe diskless machines or PCs or
Workstations with disks with only the client
software installed.


Connected to the servers via some form of a
network.


(LAN: local area network, wireless network,
etc.)

Elmasri and Navathe, Fundamentals of Database Systems,
Fourth Edition

Copyright © 2004 Pearson Education, Inc.


Slide 2
-
27

DBMS Server


Provides database query and transaction
services to the clients


Sometimes called query and transaction
servers

Elmasri and Navathe, Fundamentals of Database Systems,
Fourth Edition

Copyright © 2004 Pearson Education, Inc.


Slide 2
-
28

Two Tier Client
-
Server
Architecture


User Interface Programs and Application
Programs
run on the client side


Interface called

ODBC (Open Database
Connectivity


see

Ch 9
)
provides an
Application program interface (API) allow
client side programs to call the DBMS.
Most DBMS vendors provide ODBC
drivers.

Elmasri and Navathe, Fundamentals of Database Systems,
Fourth Edition

Copyright © 2004 Pearson Education, Inc.


Slide 2
-
29

Two Tier Client
-
Server
Architecture


A client program may connect to several DBMSs.


Other variations of clients are possible: e.g., in
some DBMSs, more functionality is transferred to
clients including data dictionary functions,
optimization and recovery across multiple servers,
etc. In such situations the server may be called the
Data Server
.

Elmasri and Navathe, Fundamentals of Database Systems,
Fourth Edition

Copyright © 2004 Pearson Education, Inc.


Slide 2
-
30

Three Tier Client
-
Server
Architecture


Common for

Web applications


Intermediate Layer called

Application Server
or

Web Server:


stores the web connectivity software and

the rules and
business logic (constraints)
part of the application used to
access the right amount of data from the database server


acts like a conduit for sending partially processed data
between the database server and the client.


Additional Features
-

Security:


encrypt the data at the server before transmission


decrypt data at the client

Elmasri and Navathe, Fundamentals of Database Systems,
Fourth Edition

Copyright © 2004 Pearson Education, Inc.


Slide 2
-
31

Classification of DBMSs


Based on the data model used
:


Traditional: Relational, Network, Hierarchical.


Emerging: Object
-
oriented, Object
-
relational.


Other classifications:


Single
-
user (typically used with micro
-

computers) vs. multi
-
user (most DBMSs).


Centralized (uses a single computer with one
database) vs. distributed (uses multiple
computers, multiple databases)

Elmasri and Navathe, Fundamentals of Database Systems,
Fourth Edition

Copyright © 2004 Pearson Education, Inc.


Slide 2
-
32

Classification of DBMSs

Distributed Database Systems
have now
come to be known as
client server based
database systems
because they do not
support a totally distributed environment,
but rather a set of database servers
supporting a set of clients.

Elmasri and Navathe, Fundamentals of Database Systems,
Fourth Edition

Copyright © 2004 Pearson Education, Inc.


Slide 2
-
33

Variations of Distributed
Environments:


Homogeneous DDBMS


Heterogeneous DDBMS


Federated or Multidatabase Systems